Automatic gain control is a well known technique for controlling the gain of an amplifier in accordance with the amplitude of the incoming input signal. AGC is widely used in radio and television receivers, as well as for controlling the amplitude of pure audio signals, as in hearing aids or microphone leveling circuits. A variety of AGC circuits have been developed in the prior art which control gain by varying either feedback resistance, amplifier bias or other circuit parameters.
One well known method of varying the gain of a bipolar transistor amplifier is to vary the DC emitter current. The power gain of a common emitter or common base amplifier rises to a maximum as the emitter current increases to a certain level, with the gain then falling as the emitter current rises past that level. Thus by automatically controlling the emitter current, automatic gain control is achieved. Another method of controlling the gain involves automatic control of the DC collector voltage. As collector voltage increases, so does the gain, until a saturation collector voltage is reached whereupon further increase in the collector voltage results in decreased gain.
In AGC circuits employing operational amplifiers, a common technique for controlling gain is to employ a variable resistor in a feedback loop. By detecting the input or output voltage in some manner and varying the feedback resistor in accordance with the detected voltage, the gain is controlled. An example of such an arrangement is presented in an article entitled "Solution AGC", by F. Baumbarmer, 73 Amateur Radio, Sept. 1990. The circuit disclosed therein utilizes an LED coupled to the op-amp output, which generates increased light intensity as the output voltage increases. A photoresistor in the op-amp feedback loop then detects the light output, thereby decreasing its resistance and hence the op-amp gain. This circuit operates as a microphone leveling circuit which enables a person to speak into a microphone standing a considerable distance away from it, while achieving substantially the same performance as if speaking directly into it.
An example of a prior art AGC circuit particularly adapted to control speech signals can be had by reference to an article entitled "Automatic Conditioning of Speech Signals", by G. Hellworth et al., IEEE Transactions on Audio and Electroacoustics, Vol. AU-16, No. 2, June 1968. In this article, an AGC circuit is disclosed employing a feedback loop which controls a variable resistor that is in shunt between the amplifier input and ground. The variable resistor is embodied as a saturated symmetrical transistor operating without collector bias. The value of this variable resistor is determinative of the gain. In the feedback loop, the output voltage is fed to a full-wave rectifier and the rectified output is then compared to a fixed voltage threshold. When the output voltage exceeds the threshold, a current source produces current pulses which, by means of control circuitry, are operable to reduce the variable feedback resistance and thereby reduce the gain. When the output voltage is below the threshold, a constant current source charges a capacitor in the feedback loop to produce a voltage which causes the variable resistance to increase, thereby increasing the gain.
In any event, one common problem with the approaches discussed above as well as with other prior art AGC loops, is that they are not particularly suitable for complete fabrication within a single integrated circuit chip. Some designs are totally discrete and would require the use of many parts to build the AGC loop. More integrated designs would use fewer parts but would still require an excessively large circuit board area for the circuits. Moreover, other prior art AGC designs draw excessive power or are overly sensitive to ambient temperature changes.
It is therefore an object of the present invention to provide an automatic gain control circuit that is particularly suitable for fabrication as a relatively simple integrated circuit.
It is another object of the present invention to provide such an AGC circuit that exhibits low power dissipation and improved temperature stability.